Im about to start insulating a log cabin with fiberglass and feel confused about which films are needed and where to place them; what flows of air/vapour should be controlled. I need clarification on the following questions. 1. What kinds of materials to use - polyethelene sheets or any other types of films? (what is the material which is an air barrier but permiable to vapour?) Location of air/vapour barrier - inside, outside, both; gaps - where are they needed and how to ventilate them? 2. In case of a house without insulation, air flow control systems water vapour from inside (and perhaps outside as well) would condense in the wall at the location of the dew point in winter causing rot and deterioration; how were those old buildings constructed without insulation/air,vapour control systems and able to remain sound for decades being continuously occupied; which secrets, tricks were used there?
Put the vapor barrier on the warm side of your insulation.
Put the house wrap (Tyvek, etc. ) on the outside as it will stop air movement but not vapor.
Perhaps you'll find some useful info at these links.
http://www.buildingscience.com/documents/digests/bsd-104-understanding-air-barriers/
http://www.buildingscience.com/documents/insights/bsi-049-confusion-about-diffusion/
http://www.buildingscience.com/documents/guides-and-manuals/gm-read-this-before-you-design-build-renovate (This is a PDF.)
Hope they help.
The secret to the old buildings was they leaked (and most still do) like a sieve, so if anything ever got wet, it would dry out. Which is great, except that you're losing all of your heat to the outside.
There are a lot of air barriers that are permeable to vapor. On the simple side some high performance builders use just OSB, with taped seams (on the inside of their walls). Then there are more fancy membranes like Intello or MemBrain. The only thing to consider is that you want to minimize penetrations through the air barrier, which makes things like wiring difficult. The easiest solution is a service cavity--another layer of 2x from 1.5 - 2.5" thick, where all of the mechanicals reside.
And fiberglass insulation is junk. If you want to use batts, at least use rock wool. Or blow dense-pack cellulose.
The previous links are excellent reading too.
Hello Ozarker, and Welcome to the Forestry Forum. :)
Thank you, Magicman!
Do what beenthere said. For the vapour barrier use 6 mil polyethylene -- thinner stuff will let a bit of vapour pass.
You are trying to prevent warm, moist air from getting into your insulation; otherwise as it passes through to the cold side you will get condensation. This means you cannot rely just on the poly. For wiring, either use special airtight electrical boxes (where they interrupt the vapour barrier) or seal any openings in regular boxes with electrical putty. Use a non-hardening putty around any other penetrations -- vents, pipes, etc. Make sure the vapour barrier fits tightly around anything that will penetrate it.
When I built my house, I stretched the vapour barrier over everthing -- windows, doors, electrical boxes -- and didn't cut it until after the paneling/sheetrock/drywall was installed. Then I cut the barrier a little smaller than the opening so it would stretch a little to fit around the penetrations.